1. Technical Field of the Invention
The present invention relates generally to strainer devices and, more particularly, to a suction strainer of modular construction which is adapted to remove entrained solids or debris from the cooling liquid in a nuclear reactor, and to reduce head loss across the strainer in the presence of liquids with such entrained solids or debris.
2. Description of the Related Art
A nuclear power plant typically includes an emergency core cooling system that circulates large quantities of cooling water to critical reactor areas in the event of accidents. A boiling water reactor or BWR commonly draws water from one or more reservoirs, known as suppression pools, in the event of a loss of coolant accident. More particularly, water is pumped from the suppression pool to the reactor core and then circulated back to the suppression pool in a closed loop. A loss of coolant accident can involve the failure of reactor components that introduce large quantities of solid matter into the cooling water, which entrains the solids and carries them back to the suppression pool. For example, if a loss of coolant accident results from the rupture of a high pressure pipe, quantities of thermal insulation, concrete, paint chips and other debris can be entrained in the cooling water.
In contrast to a BWR, a pressurized water reactor or PWR, after a loss of coolant accident, typically draws cooling water from a reactor water storage tank and, after a signal, shuts off the flow from the storage tank and recirculates this water through the reactor. In this regard, the pressurized water reactor has a containment area that is dry until it is flooded by the occurrence of an accident, with the emergency core cooling system using a pump connected to a sump in the containment area to circulate the water through the reactor. Nevertheless, the water that is pumped in the event of an accident will also usually contain entrained solids that typically include insulation, paint chips, and particulates. Thus, in both types of reactors (i.e., boiling water reactors and pressurized water reactors), cooling water is drawn from a reservoir and pumped to the reactor core, with entrained solids or debris potentially impairing cooling and damaging the emergency core cooling system pumps if permitted to circulate with the water.
In recognition of the potential problems which can occur as a result of the presence of entrained solids or debris in the coolant water of the emergency core cooling system, it is known in the prior art to place strainers in the coolant flow path upstream of the pumps, usually by immersing them in the cooling water reservoir. It is critical that these strainers be able to remove unacceptably large solids without unduly retarding the flow of coolant. In this regard, the pressure (head) loss across the strainer must be kept to a minimum. Strainers are commonly mounted to pipes that are part of the emergency core cooling system and that extend into the suppression pool or sump, with the emergency core cooling system pumps drawing water through the strainers and introducing the water to the reactor core. There has been considerable effort expended in the prior art in relation to the design of strainers to decrease head loss across the strainer for the desired coolant flow. Existing strainers often include a series of stacked perforated hollow discs or flat perforated plates and a central core through which water is drawn by the emergency core cooling system pump. The perforated discs or plates prevent debris larger than a given size from passing the strainer perforations and reaching the pumps.
As is apparent from the foregoing, large amounts of fibrous material can enter the circulating coolant water in the event of a reactor accident. This fibrous material, which often originates with reactor pipe or component insulation that is damaged and enters the emergency core cooling system coolant stream in the event of a loss of coolant accidents indicated above, typically accumulates on the strainer surfaces and captures fine particulate matter in the flow. The resulting fibrous debris bed on the strainer surfaces can quickly block the flow through the strainer, even though the trapped particulates may be small enough to pass through the strainer perforations. More particularly, the debris accumulates in a fluffy density in and on the strainer until the strainer becomes completely covered with a fiber and particulate debris bed. Once this occurs, the strainer loses its complex geometric surface advantages and becomes a simple strainer. Hours to days later, some debris typically dissolves into solution and interacts with chemicals present in the containment. At the same time, containment temperatures are trending down. This phenomenon causes certain chemical precipitates to form which eventually make their way to the strainer. Once they reach the strainer surface, the pressure drop across the strainer typically dramatically increases.
The prior art has attempted to address the above-described flow blockage effect by making the strainer larger, the goal being to distribute the trapped debris over more area, reducing the velocity through the debris bed, and further reducing the head loss across the strainer as a whole. This solution, however, is often undesirable since the available space in a reactor for a suction strainer is usually limited, and further because larger strainers are typically more costly. As a result, the situation sometimes arises wherein the expected debris load after a loss of coolant accident can dictate a need for strainers that are too large for the space allotted for them in the containment area. Moreover, large strainers are often more difficult work with and thus more costly to install. In addition, prior art emergency core cooling system strainers have been constructed in ways that make them somewhat expensive to fabricate.
The present invention addresses the aforementioned needs and overcomes many of the deficiencies associated with existing nuclear power plant strainer designs providing a strainer design which is specifically suited to reduce the differential pressure experienced across the strainer in nuclear power plants with medium to high fiber loads after chemical precipitate formation. Various features and advantages of the present invention will be described in more detail below.